The homeobox gene sine oculis (so) is required for the formation of the Drosophila eye. As a transcription factor, So functions at multiple stages in eye development including tissue specification, morphogenesis and neuronal differentiation. The dynamic expression of So during embryonic and larval stages suggests that So plays multiple roles not only in eye development, but in other tissues as well. Thus, it is likely that tissue-specific cofactors modulate its activity through direct and indirect interactions. To identify potential partners, a yeast two-hybrid screen was performed using the Six domain, a region of Sine oculis known to be involved in protein-protein interactions. Two novel proteins were found in addition to several known factors. Both are expressed in the developing eye in mutually exclusive domains that overlap with so expression. Thus, these factors may interact with So in vivo and may modulate its activity at different steps of eye development. The first aim of this research proposal is to determine how these factors influence the regulation of transcription by So. The second aim is to elucidate the function of the novel factors in eye development. The third aim involves detailed phenotypic analyses of these two factors and So to define their roles in the various regulatory pathways that control the different stages of eye development. Considering the conservation the Six gene family, this work will hopefully provide new insights into the molecular mechanisms of the developing vertebrate eye.